Metal carbide formation on carbon fibers



Dec. 5, 1967 c. 1.. GUTZEIT 3,356,525

METAL CARBIDE FORMATION ON CARBON FIBERS Filed Nov. 18, 1963 900 /000/292 I472 I652 I832 ma'c 200 2/2; 392

INVENTOR. Carlos A. uizeif l/r ornays United States Patent 3,356,525METAL CARBIDE FORMATION 0N CARBON FIBERS Carlos L. Gntzeit, Long Beach,Calif., assignor to Hitco Corporation, a corporation of California FiledNov. 18, 1963, Ser. No. 324,477 12 Claims. (Cl. 117-46) The presentinvention generally relates to improvements in fibers and moreparticularly relates to improved oxidation-resistant carbon fibers andto a method of improving the oxidation resistance of carbon fibers.

Amorphous carbon fibers offer improved properties for various uses, incontrast to some older types of fibers. Thus, carbon fibers have beenwidely accepted in the missile propulsion field inasmuch as they combinethe excellent ablation characteristics of graphite with a lower thermalconductivity than graphite.

Carbon fibers are particularly suitable for high temperature use in thatthey sublime at an extremely high temperature, in excess of 6600 F., andbecause their strength increases with temperature. Moreover, the carbonresists chemical attack except highly oxidizing substances at elevatedtemperatures, for example, above 800 F. In addition, the electricalresistance of carbon fibers can vary according to their diameter,surface area, etc., so that such carbon fibers can act either aselectrical insulators or as electrical conductors for specialapplications.

Since carbon fibers are flexible and readily formable, and since they donot smear or rub oif as do graphite fiber and cloth, they can be readilyand conveniently fabricated into a wide variety of products, includingcloth or fabric, sleeving, roving, tape, felt, batts and bulk fiberproducts, using fibers of uniform or of varying lengths. The fibers areavailable in diameters ranging up to about 0.005 inch or more and inhigh purity. For example, a typical amorphous carbon fiber analysis isthe following:

Carbon-95.2% by weight; hydrogenl.0% by weight; oxygen3.3% by weight;impurities0.5% by weight (composed mainly of nickel, copper and tin,with trace quantities of calcium, iron, silicon, magnesium andtitanium).

Carbon fibers can be prepared, for example, in accordance with theteachings of copending US. patent application, Ser. No. 160,605, nowPatent No. 3,294,489, filed Dec. 19, 1961 and entitled Process forPreparing Carbon Fibers, of which Richard D. Millington et al. are theinventors, said application having been assigned to the assignee of thepresent invention. As described in that patent application,non-thermoplastic cellulosic fiber, such as rayon, wool, etc., isscoured to remove any finishing material, rinsed free of scouringsolution, dried and then fired in a closed atmosphere at controlledsuccessively higher temperatures to drive ofi volatiles and carbonizethe same without destroying their fibrous nature. During such treatment,the temperature increases from about 300 F. to less than about 1000 F.in increments of 20- 50 F. at 8-30 hour intervals. Thereafter, a flashfiring operation under non-oxidizing conditions is carried out at, forexample about 1800-2000 F. to drive off remaining volatiles and heatshrink the fibers, after which the fibers are cooled to below 300 F.under non-oxidizing conditions. Without such controlled sequentialheating and flash firing of the fibers during processing, the carbonfibers would tend to disintegrate into separate masses, and laminatedstructures subsequently built therefrom would be much more likely todelaminate during use at elevated temperatures. Furthermore, the flashfiring decreases the reactivity of the carbon fibers.

3,356,525 Patented Dec. 5, 1967 Although carbon fibers produced as perthe foregoing description are resistant to most kinds of chemicalattack, erosion and change of structure, except at extremely hightemperatures at which the fibers sublimate, such fibers still aresubject to pronounced deteriorations and rapid Weight loss when exposedto elevated temperatures in excess of about 800 F. in the presence of anoxidizing medium. Under such circumstances, the carbon of the fiberstends to react with the oxygen of the oxidizing medium (e.g.

atmosphere) to form the carbon oxides, carbon monoxide and carbondioxide. Accordingly, the basic structure of the fiber volatilizes andis lost.

Accordingly, it is a principal object of the present invention toprovide improved oxidation-resistant carbon fibers.

It is also an object ofthe present invention to provide a method ofimproving the oxidation resistance of amorphous carbon fibers.

It is still a further object of the present invention to provide asimple, effective, low cost method of improving the oxidation resistanceof amorphous carbon fibers at temperatures in excess of 800 F.

These and other objects are accomplished, in accordance with the presentinvention, by providing an improved method which substantially increasesthe oxidation resistance of amorphous carbon fibers at elevatedtemperatures, for example, in excess of about 800 F. It Will beunderstood that the term car-bon fibers herein is directed only toamorphous or essentially amorphous carbon fibers, as contrasted withgraphite fibers. The latter fibers have characteristics distinctlydifferent from those of carbon fibers and treatment thereof is not apart of the present invention. The method is simple, inexpensive, rapidand effective.

In accordance with the method, carbon fibers are used which havepreviously been subjected to controlled sequential firing operations toremove volatiles and which also have been previously subjected to flashfiring operations at temperatures of, for example, about 2000 F. or thelike, so that remaining volatiles have been removed and the fibers havebeen heat shrunk. These car-bon fibers are-coated with suitable organiccarbon-containing material which also contains metal selected from thegroup consisting of zirconium, chromium, titanium, nickel and mixturesthereof. The coating step is carried out so that the fibers aresubstantially completely enclosed in the material, after which thefibers are treated in a manner to form refractory carbides of the metalsubstantially completely from the carbon and metal of the material.

The carbide is, in accordance with the present method, uniformlydistributed over substantially the entire surface of each of the fibers,and acts to protect the same. Moreover, the carbide impregnates thefiber surface, i.e., is disposed in the surface irregularities of thefibers and is permanently bonded to the fibers. When the carbide encasedfibers are exposed to an oxidizing environment, such as air at aboveabout 800 F., the carbide of each fiber is converted to thecorresponding refractory oxide. The refractory oxide, in turn, does notreact with the atmosphere and, accordingly, provides a strong,protective, continuous oxidation-resistant covering over the carbonfibers. Rapid deterioration of the carbon fiber in such environment isthereby avoided. Moreover, the refractory oxide not only functions as achemical barrier, resisting oxidative deterioration, but also acts as athermal shield, improving the overall durability of the carbon fiber.

As a specific example, car-bon cloth, which has been manufactured byscouring, rinsing and drying rayon cloth, heating the cloth in a closedatmosphere from 300 up to 1000 F. in 50 increments at 8 hour intervals,and then flash firing the cloth at about 2200 F. for 10 seconds, istreated according to the following procedure:

The resulting flash fired carbon cloth is immersed in an impregnatingsolution containing 175 gm. of chromium acetate, i.e., suflicient toprovide approximately weight percent concentration of chromium carbidein the finished cloth, the weight being based on the metal itself. Thecarbon cloth absorbs about 0.8 ml. (milliliter) .of impregnatingsolution per gm. (gram) of the cloth. The carbon cloth is maintained inthe solution for 20 minutes, then withdrawn from the solution, drained,air dried at about 250 F. and calcined in a non-oxidizing atmosphere, inthis instance nitrogen, although an inert gas such as argon, krypton,neon or the like can be used, for 15-20 seconds at about 2200 F., atemperature sufliciently high to decompose the acetate to chromiumcarbide. The in situ formed carbide wholly and tightly encloses eachcarbon fiber, is bonded to the surface thereof and fills the surfaceirregularities and pores of the carbon fiber. Thereafter, the treatedcloth is cooled to ambient temperature in the non-oxidizing atmosphere.Further objects and advantages of the present invention will be apparentfrom a study of the following detailed description and the accompanyingdrawings, of which:

The single figure is a graph depicting oxidation resistance of carbonfibers treated by the method of the invention.

Now referring more particularly to the steps of the method of thepresent invention, carbon fibers in any suitable form, for example, inthe form of a mat, batt, felt, fabric, sleeving, roving, tape, or asbulk fibers or the like are treated so as to increase their oxidationresistance.

The carbon fibers to be treated can be produced in any suitable manner,such as that more particularly set forth in the previously describedcopending United States patent application, Ser. No. 160,605, Millingtonet al. In any event, the carbon fibers to be treated should already bein finished form, that is, they should have been subjected previously toa flash firing or comparable operation at, for example, about 1800-2000P. so as to have had essentially all volatiles removed therefrom and soas to have been heat shrunk.

This is a requirement inasmuch as if the treatment were applied tocarbon fibers which still contained some moisture or other comparablevolatiles, release of such moisture or other deleterious volatiles fromthe carbon fibers during the calcining step in the present method wouldhave the effect of hydrolyzing (in the case of moisture) or of otherwiseadversely affecting the carbide being formed on the surfaces of thefiber so as to cause the loss of carbide from such surfaces.Accordingly, only carbon fibers which are already in the flash fired,heat shrunk, substantially completely devolatilized condition are usedin the present method.

Such carbon fibers are contacted in the present method with a selectedagent consisting essentially of heat decomposable material containingcarbon and metal selected from the group consisting of nickel, chromium,zirconium, titanium and mixtures thereof. Only these metals have beenfound to be fully effective in preparing the desired carbide coating atrelatively low temperature on the surface of the carbon fibers, inaccordance with the present method.

It is important to note that heat decomposable material is used whichyields a carbon-containing residue at calcining temperature. Thus, thematerial itself serves as the source of carbon for carbide formation sothat the relatively small diameter carbon fibers need not and do notmaterially contribute carbon to the carbide-forming reaction.Accordingly, the integrity of the fine diameter carbon fibers isretained while still providing in situ carbide formation on the surfacesthereof. If, instead, the source of carbon for the in situ carbideformation were 4 the carbon of the carbon fibers, then such in situcarbide formation would result in a depreciation in the integrity andmass of the carbon fibers. Inasmuch as the carbon fibers usually are ofrelatively small diameter and, accordingly, large surface area, in suchinstance an appreciable portion of the total mass of the carbon fiberswould be involved in carbide formation from the substance thereof. Uponsubsequent use of such fibers in an oxidizing environment at elevatedtemperature, for example at above 800 F., a substantial proportion ofthe substance of the carbon fibers would be lost by conversion of thecarbide to the corresponding refractory oxide and release of the carbonfrom the carbide in the form of carbon oxide. Accordingly, the mainobject in coating the carbon fibers would be defeated, i.e. to protectthe substance or mass thereof against oxidation and loss from the fibersin the form of volatile carbon oxides.

In view of the above, the present method provides a source of carbonexternal of the carbon fibers themselves and disposed in intimatecontact with the selected metal in the treating agent so as to assureeffective carbide formation from the material of the treating agent.

The selected metals, nickel, titanium, zirconium and chromium, are thosewhich will readily react to form protective carbides by decomposition oforganic compounds containing the same at relatively low temperature andwhich will form carbides which do not detract from desiredcharacteristics of the carbon fibers. With the heat decomposable organiccompounds containing the selected metals, decomposition can take placeso as to provide the carbide in situ without debilitating the carbonfibers. Moreover, the relatively low temperatures represent a saving inprocessing costs. Thus, when organic compounds in accordance with theinvention are used, decomposition temperatures of the order of about1000 C. can be effectively employed in contrast to the usual carbonizingtemperatures of at least about 1500 C.2000 C.

Any suitable carbon-containing compound which also contains metalselected from the group consisting of chromium, zirconium, titanium,nickel and mixtures thereof can be used. However, those compounds whichare readily soluble in a suitable inexpensive solvent or other medium,such as water, are preferred. So also are those compositions which canbe concentrated to viscous syrups which dry to glass-like, substantiallycontinuous smooth coatings in contrast to those compounds which readilycrystallize and, accordingly, have a tendency to provide a discontinuousisland configuration coating, especially over the surfaces of smalldiameter fibers, such as carbon fibers.

Thus, it is preferred to use an organic material or compound such as anacetate or formate, for example chromium acetate, zirconium acetate,zirconium formate, nickel acetate, nickel formate, titanium. oxidehydrosol, stabilized by acetic acid, and the like.

Chromium acetate and zirconium acetate generally are considered to beacetate complexes rather than true compounds. They can be concentratedto viscous syrups which can be applied to smoothly, uniformly and evenlycoat the entire exposed area of the carbon fibers to be treated and caneasily penetrate pores in the carbon fiber and fill surfaceirregularities, so as to improve bondability of the compound to thefiber. In other words, such compounds readily wet and adhere to thecarbon fibers. So also do carbides formed in situ from these compounds.

Nickel acetate is usually crystalline but again can be dissolved in asuitable solvent, such as Water, and can be used in that form, Titaniumoxide hydrosol stabilized by acetic acid is a colloid which is stableover a limited range of conditions and which sets to an all embracinghydrogel when heated. Selected formates are also preferred, for example,nickel formate which has the formula is a dark green tanning liquidwhich can also be used in the present invention. It will be understoodthat any other suitable heat decomposable organic compound which yieldscarbide of nickel, chromium, zirconium, titanium or mixtures thereofupon heating, can also be used in accordance with the present invention,for example, selected organo-metallic compounds such as metal chelatingagents.

The treating agent is usually disposed in a suitable medium, such as adispersant, solvent or the like so that it can'efiectively uniformlycontact and penetrate into the carbon fibers to be treated. For example,zirconium acetate, chromium acetate, nickel acetate, nickel formate,chromium formate and certain other organic compounds are Water soluble.The water can be subsequently removed by evaporation, as by drying theimpregnated coated fibers. In the event another solvent or dispersant isused, to which the carbon is inert, for example, ethyl alcohol, etc.,depending on the treating agent, a drying step can also be used toremove such solvent from the coating.

The treating agent is present in the medium in any suitableconcentration. In this regard, the minimum concentration used is thatwhich provides an essentially continuous refractory carbide coating overthe entire exposed surface area of the carbon fibers to be treated uponone or a series of immersions. It has been found that, depending uponthe extent of surface area of the fibers, the irregularities in thesurface of the fibers and other factors, the minimum concentration ofthe treating agent will necessarily vary so as to vary the amount ofcarbide coating enclosing the carbon fibers. Usually, a carbide coating,the metal of which weighs at least about 0.5 percent that of the finalproduct (carbide plus the carbon fibers) will be sufiiciently thick andcontinuous to effectively protect the underlying carbon fiber againstoxidative deterioration at temperatures in excess of 800 F. Carbideconcentrations of up to Weight percent or more can be used, based uponthe weight of the metal of the carbide in the finished product. However,concentrations greater than about 10 weight percent may result in asubstantial change in one or more properties of the carbon fibers, forexample, the general appearance and texture of carbon cloth. Moreover,concentrations of carbide of about 2-4 percent, based on the metalthereof, are preferred for maximum efficiency of oxidation resistanceand minimum difliculties.

Carbide concentrations of over about 6 weight percent of the finishedproduct offer little improvement of oxidation resistance overconcentrations of somewhat less than 6 weight percent. Moreover, thereis some small tendency of the product during processing to exhibitincreasing brittleness as the carbide concentration increases above 6weight percent. The increased brittleness and stiffness of the carbonfibers is due, apparently, at least in some instances to the glassynature of the selected dehydrated metal complex used as the treatingagent. The brittleness disappears from the fiber when the metal complexis thermally decomposed to the carbide. However, it is simpler, easierand more economical to use lower concentrations of the metal and avoidthe inconvenience and possibility of mechanical rupture due to thestiffness and brittleness of the carbon fibers containing concentrationsof the metal complex in excess of about 6 weight percent. Accordingly,for most purposes, the maximum concentration of the carbide in thefinished product will be about 6 weight percent, based on the weight ofthe metal of the carbide.

' In the present method, the carbon cloth is contacted with the treatingagent in any suitable manenr, e.g. immersion in the treating agent or amixture, dispersion or solution, etc. of the treating agent and asuitable medium. Alternatively, the fibers can be sprayed with thetreating agent or treating agent-medium or can be passed through 6 azone wherein the treating agent or treating agent-medium mixture hasbeen vaporized, etc. Still alternatively, the treating agent can bepainted on, poured on or otherwise contacted with the carbon fibers. Thecontacting is 3 carried out so as to impregnate the carbon fibers andcoat,

cover or encapsulate essentially all exposed surfaces thereof with thetreating agent.

After the desired concentration of treating agent has been absorbed bythe carbon fibers, the carbon fibers are removed from contact with theremaining treating agent, as by withdrawing the carbon fibers from anaqueous solution or bath of the treating agent. The carbon fibers arethen treated to remove excess treating agent, as by draining the same,and thereafter the carbon fibers are dried or otherwise freed oftreating agent medium, if any, and treated so as to bond the treatingagent to the surface of the fibers as an enclosing protective cover andso as to set the treating agent. For example, water solvent forzirconium acetate treating agent is removed from the treating agentcontained on the surfaces of the carbon fibers, as by air drying at, forexample, about 250 F. for a period of about one-half hour in circulatingair. The treating agent thereupon sets to a solid glassy covercomprising zirconium acetate.

Carbon fibers thus impregnated, coated and wholly enclosed within thetreating agent are then heat treated (cal cined) to convert the treatingagent to the desired carbide or carbides by decomposition of thetreating agent to a carbon and metal-containing carbide-forming residue.In this regard, the dried carbon fibers containing a uniform coating ofthe treating agent disposed over the surface thereof are calcined undernon-oxidizing conditions, e.g. nitrogen, hydrogen, helium, argon, etc.at a temperature in excess of about 1000 C. (about 1800 F.) for asufficient period of time to substantially completely decompose thecarbon-containing treating agent and form the desired selected carbideor carbides in situ on the fibers. The calcining temperature can besubstantially lower than that temperature which would be required toreact the pure refractory met-a1, or metal hydride or oxide with carbonto form the carbide (carburizing temperatures). It will be noted thatthe calcining temperature at which the carbide is formed in situ in thepresent method should be maintained below that at which the carbon isconverted into graphite fiber (graphitizing temperature), because thethermal conductivity of graphite fiber is substantially higher than thatof carbon fiber and, for high temperature applications, less desirable.

The thus prepared selected carbide coated carbon fiber product is thencooled to ambient temperature and is then ready for use. The coolingstep can take place under any suitable conditions, for example,non-oxidizing conditions or alternatively, oxidizing conditions,inasmuch as the carbide layer effectively protects the carbon fiberagainst oxidation at elevated temperatures. Thus, the cooling step caneither be carried out in the presence of hydrogen, nitrogen, argon,krypton, xenon, helium or the like, or in a vacuum or in the presence ofoxygen or an oxygen-containing atmosphere, such as air.

The finished carbide coated carbon fibers are now ready for use. Theyare particularly suitable as thermal insulation, and can be exposed tohigh temperatures under oxidizing conditions wherein unprotected carbonfibers would rapidly be oxidized and rapidly lose mass and utility. Thecontinuous carbide coating bonded to the surface of the carbon fibersand filling the surface pores and irregularities thereof is effectiveboth as an oxidation-resistant barrier and as a heat shield for thecarbon fibers. In this regard, the carbides exhibit improved insulatingqualities over the carbon fibers themselves. Moreover, zirconiumcarbide, titanium carbide, chromium carbide and nickel carbide areconverted to the respective oxides upon exert to. oxidizing conditionsare, formed in situ and tightly.

Example I Carbon cloth is treated according to the present method. Thecarbon cloth has the following chemical analysis:

Percent by wt.

Carbon 95.2 Hydrogen 1.0

Oxygen 3.3 Ash 0.5

An aqueous solution containing a sufficient concentration of zirconiumacetate to provide 4 weight percent of zirconium carbide, calculated aszirconium, after calcining carbon cloth impregnated with the solution,is used as the treating solution. The carbon cloth is immersed thereinand the cloth absorbs approximately 0.8 m1. of solution per gram ofcarbon cloth over a 30 minute period. Thus, every 800 ml. of thesolution contains approximately 192 grams of zirconium acetate havingthe formula The cloth is then Withdrawn from the solution, drained ofexcess aqueous zirconium acetate solution and then is passed through adryer operating at about 275 F. until dry, in about 20 minutes. Thus,excess Water is removed from the zirconium acetate and the compound isdried into a glassy solid state as a continuous smooth layer bonded toexposed surfaces of the carbon fibers and impregnating and filling allsurface pores and surface irregularities.

The dry, coated cloth is then passed to a furnace containing a nitrogenatmosphere and is heated slowly over a period of about 5 hours to atemperature of 1000 C. and maintained at this temperature for a periodof about 8 hours. At the end of this time the heating unit of thefurnace is shut otf and the cloth is allowed to cool to ambienttemperature in the nitrogen atmosphere. During such firing treatment at1000 C. in the furnace, the volatiles of the zirconium acetate areremoved and the zirconium acetate is decomposed to zirconium carbide.The zirconium carbide formed in situ is bonded to the surfaces of thecarbon fibers, filling the surface pores and irregularities andeffectively protecting the fibers against erosion due to oxidation. Thecarbon of the carbide is substantially completely derived from thecarbon of the acetate rather than the carbon of the carbon fibers.Accordingly, the mass of the carbon fibers is retained substantiallycompletely intact during the in situ carbide formation.

Upon examination, the finished fibers are found to have substantiallyimproved oxidation resistance and somewhat lowered thermal conductivity,in contrast to untreated carbon fibers of the same type. In all otherrespects, the treated carbon fibers are substantially identical to theuntreated carbon fibers.

Example 11 Carbon cloth is treated in accordance with the method setforth in Example 1, except that an aqueous solution of zirconium formateis used instead of the aqueous solution of zirconium acetate. Moreover,the concentration of zirconium formate is such as to provide in thefinished carbon cloth a concentration of about 2 percent, by weight, ofzirconium in the form of the desired zirconium car- In a parallel seriesof tests, a chromium carbide coating is formed on flash-fired carbonfibers in accordance with the method of Example I, but utilizing anaqueous solution of chromium acetate in a concentration to provide inthe finished product a chromium carbide concentration of about 2 percent(based on the chromium) by weight of the finished product.

The zirconium carbide coated carbon fibers (2 wt.-percent zirconium) andalso the chromium carbide coated fibers (2 wt.-percent chromium) exhibitthe same general appearance and other characteristics as the uncoatedcarbon fibers, except for substantially improved oxidation resistance attemperatures in excess of 800 C.

The single figure of the accompanying drawings graphically depicts theresults of oxidation resistance tests performed on fibers prepared bythe present method. In each instance, the temperature of one gramsamples of the fibers was raised at the rate of 10 C. per minute toapproximately 900 C.l000 C. While the fibers were ex posed to airflowing at the rate of 1 s.c.f. (Standard cubic foot) per hour. Theweight loss for the fibers was measured at various times during thetests. Line A indicates the progressive weight loss during the testperiod of uncoated flash-fired carbon cloth, while Line B indicates theprogressive weight loss during the test period for the zirconium carbidecoated carbon fibers, and Line C indicates the same type of data for thechromium carbide coated carbon fibers. As clearly indicated in thesingle figure, during the test period the zirconium carbide treatedcarbon fibers (Line B) exhibited substantially improved oxidationresistance in contrast with the uncoated carbon fibers (Line A). Thissubstantially improved oxidation resistance became particularly apparentat temperatures in excess of about 900 C. Moreover, the chromium carbidecoated carbon fibers (Line C) have even more improved oxidationresistance throughout substantially the entire test temperature range.Accordingly, the single figure clearly indicates that substantiallyimproved oxidation resistance is afforded to carbon fibers by providingthe same with a coating of selected carbide.

Similar tests have been run on nickel carbide coated carbon fiberscontaining approximately 5 weight percent nickel, chromium carbidecarbon fibers containing approximately 6 weight percent chromium,titanium carbide coated carbon fibers containing 2 weight percenttitanium and untreated carbon cloth, i.e. carbon cloth containinguncoated carbon fibers. The tests have been run in accordance with thepreceding criteria, i.e. by exposing the one gram samples of the variousfibers to an air flow of 1 s.c.f. per hour and a temperature rise of 10C. per minute over a temperature range of from about ambient temperatureto about 1000 C. At the end of the test period, the weight percentresidue, based upon the initial weight concentration of each of thetypes of samples, was determined. It was found that the carbon clothwhich had not been protected by a carbide layer had decreased in mass toonly 2.15 weight percent that of the original mass. The titanium carbidecoated fibers (containing about 2.8 percent of titanium, based on thetotal fiber weight) exhibited substantially improved oxidationresistance in that the weight of the residue was approximately 33.2weight percent that of the initial fibers. The

while the chromium carbide coated carbon fibers were present in theresidue in a concentration of 37.4 weight percent that of the initialfibers.

Example II] An aqueous solution of nickel formate is prepared into whichsolution flash fired carbon cloth is immersed. The solution has aconcentration of nickel formate sufii- 'cient to provide inthe finisheddried and calcined carbon cloth product a nickel concentration ofapproximately weight percent. Thus the nickel formate solution containsapproximately 264 grams of nickel formate per liter of solution. A 30liter volume of the solution is used and a 10,000 gram sample of carboncloth is disposed therein. The carbon cloth is left immersed in thenickel formate solution for a period of about one-half hour, after whichit is withdrawn from the solution, drained, dried at about 290 F. andthen calcined at about 1200 C. for about 2 hours in a nitrogenatmosphere. It is then cooled to ambient temperature in the nitrogenatmosphere over a period of about 3 hours.

The nickel carbide coated carbon cloth is examined and found to have thecarbide coating wholly enclosing and tightly bonded to all of theexposed surfaces of the carbon fibers of the cloth. Moreover, thecarbide fills the pores, depressions, and surface irregularities of thecarbon fibers. When the carbide-containing cloth is exposed to anoxygen-containing atmosphere, such as air, at a temperature in excess of800 F. the carbide is found to have been converted to nickel oxide, andit is further found that the nickel oxide coating is continuous andfully protective over the surfaces of the carbon fibers. The oxidecoating is essentially inert at such elevated temperature in theoxygen-containing environment and also acts as a thermal shield.

In a parallel series of tests, samples of titanium carbide coated carbonfibers are prepared and tested. In this regard, flash fired carbonfibers are first immersed in cloth form in a bath containing titaniumoxide hydrosol, stabilized by acetic acid, as the treating agent.

The titanium oxide hydrosol is obtained by addition of ethylorthotitanate to an agitated, cold, 25 percent by weight, aqueous aceticacid solution to obtain an approximately percent colloidal solution oftitanium oxide. The solution forms an all-embracing gel if warmedappreciably above room temperature. The solution is analyzed fortitanium oxide concentration by drying and igniting an aliquot samplethereof to form titanium oxide. The solution is then diluted with 25percent aqueous acetic acid solution to 4.25 weight percent Ti0 content,which is equivalent to 2.5 weight percent metallic titanium. Whensubsequently used to impregnate carbon cloth, this treating agentproduces a coating of 2 weight percent Ti as the acetate stabilizedoxide.

The cloth is withdrawn from immersion in the solution after about onehour, drained, and then dried at a temperature of about 250 F. On dryingthis treating agent on the cloth produces a gelatinous coating whichretains acetic acid until it is calcined. The calcining is carried outin hydrogen at a temperature in excess of 1000 C. but below about 1100C. for a period of approximately 8 hours, that is, until substantiallyall of the treating agent decomposes into and forms titanium carbide asa continuous protective layer over the surface of the carbon fibers ofthe cloth. This finished product is then cooled to ambient temperaturein hydrogen and it is found that the titanium carbide coating (2 percenttitanium, by weight of the finished product) is uniform and tightlybonded to the carbon fiber surface and that it protects all surfaces ofthe car-bon fibers of the cloth. Such carbide coating effectivelyincreases the oxidation resistance of the cloth.

The foregoing examples clearly illustrate that the method of the presentinvention results in improved carbon fibers having increased oxidationresistance, particularly at elevated temperatures. The fibers can beprepared in a simple, effective and rapid manner utilizing readilyavailable materials. The finished fibers have a bonded outer coating ofcarbide selected from a group consisting of titanium carbide, zirconiumcarbide, chromium carbide, nickel carbide and mixtures thereof, whichcarbide effectively protects the underlying carbon fibers against lossof mass when the fibers are expose-d to oxidizing con ditions. Upon suchexposure, the carbide forms the cor- 10 responding oxide which is thenstable against oxidation and forms a heat shield around the fibers.

Accordingly, a new improved product and a new method are provided. Thepresent method is specifically directed to treatment of carbon fibersand is contrasted to other possible ways of forming carbides, in thatthe integrity of the small diameter carbon fibers is maintained duringcarbide formation in situ by the present method by providing a source ofcarbon other than the fibers. Moreover, the carbide-forming reactiontakes place merely by decomposition of an organic compound at relativelylow temperature. Other advantages of the present invention are as setforth in the foregoing.

Various modifications, changes, rearrangements and alterations can bemade in the present method and in the product provided thereby. All suchmodifications, changes, rearrangements and alterations as are within thescope of the appended claims form a part of the present invention.

What is claimed is:

1. A method of improving the oxidation resistance of amorphous carbonfibers, which method comprises covering the surfaces of carbon fiberswith and bonding to said surfaces a layer of heat decomposable organicmaterial containing carbon and metal selected from the group consistingof zirconium, chromium, nickel, titanium, and mixtures thereof, saidcarbon fibers having been heat shrunk at about 1800-2200 F., andthereafter decomposing said material under non-oxidizing conditions andat elevated temperatures to carbide of said metal while substantiallypreserving the carbon of said fiber, said temperature being below thatat which graphitization occurs, whereby said fibers have improvedoxidation resistance at temperatures in excess of 800 F.

2. A method of improving the oxidation resistance of amorphous carbonfibers, which method comprises substantially wholly enclosing amorphouscarbon fibers within a layer of heat decomposable organic materialcontaining carbon and metal selected from the group consisting ofzirconium, chromium, nickel, titanium and mixtures thereof, said carbonfibers having been heat treated at below about 1000 F. and then heatshrunk at about 1800-2200 F., bonding said layer to the surface of saidfibers and heat decomposing said layer to carbide of said metal undernon-oxidizing conditions at a temperature in excess of about 1000 C. butbelow graphitization temperature while maintaining the carbon of saidfiber in unaltered form, whereby said fibers have improved oxidationresistance at temperatures in excess of 800 F.

3. The method of claim 2 wherein the metal of said carbide is present insaid fibers in a concentration of between about 0.5 and about 10percent, by Weight of said fibers.

4. The method of claim 3 wherein the metal of said carbide is present insaid fibers in a concentration of lgtween about 2 and about 4 percent byweight of said ers.

5. A method of improving the oxidation resistance of amorphous carbonfibers, which method comprises impregnating and substantially whollyenclosing the exposed surfaces of amorphous carbon fibers with heatdecomposable organic material containing carbon and metal selected fromthe group consisting of zirconium, chronuum, titanium, nickel andmixtures thereof, said material disposed in a suitable medium in asuificient concentration to provide, upon subsequent decompositionthereof, carbide of metal on the surface of said fibers in an oxidationresistant-enhancing amount, removing said medium and bonding saidmaterial to said fibers, and thereafter calcining said fibers at atemperature in excess of about 1000 C. but below graphitizationtemperature under non-oxidizing conditions while maintaining the carbonof said fibers in unaltered form, whereby said material is decomposed tocarbide of said metal and whereby the oxidation resistance of saidfibers at temperatures in excess of about 800 'F. is increased.

6. The method of claim wherein the metal of said carbide is present in aconcentration of between about 0.5

and about percent, by weight of said fibers.

7. The method of claim 6 wherein said metal of s-aid carbide is presentin a concentration of between about 2 and about 4 percent, by weight ofsaid fibers.

8. A method of improving the oxidation resistance of amorphous carbonfibers, which method comprises impregnating and wholly enclosing theexposed surfaces of amorphous carbon fibers in an organic compound heatdecomposable to a carbonaceous residue containing metal selected fromthe group consisting of zirconium, chromium, nickel, titanium andmixtures thereof, said compound being disposed in a suitable medium,evaporating said medium from said compound and setting said compound onsaid fibers so as to bond the same into a continuous adherent coveringon the surface of said fibers, heating said fibers to above about 1000C. but below graphitizing temperature under non-oxidizing conditions andmaintaining said fibers at said temperature under said conditions untilcarbide of said metal is substantially completely formed from saidcompound while the carbon of said fibers remains unaltered, whereby theoxidation resistance of said fibers is increased.

9. The method of claim .8 wherein an aqueous solution of zirconiumacetate is used to impregnate said fibers and wherein said impregnatedfibers are dried below about 300 F. and then calcined to provide azirconium carbide protective layer bonded to the surface of and Whollyenclosing said fibers, the Zirconium of said carbide being present in aconcentration of between about 0.5 and about 10 percent, by weight ofsaid fibers.

10. The method of claim 8 wherein an aqueous solution of chromiumacetate is used to impregnate said fibers and wherein said impregnatedfibers are dried below about 300 F. and then calcined to provide achromium carbide protective layer bonded to the surface of and whollyenclosing said fibers, the chromium of said carbide being present in aconcentration of between about 0.5 and about 10 percent, by weight ofsaid fibers.

11. The method of claim 8 wherein an aqueous solution of nickel formateis used to impregnate said fibers and wherein said impregnated fibersare dried below about 300 F. and then calcined to provide a nickelcarbide protective layer bonded to the surface of and wholly enclosingsaid fibers, the nickel of said carbide being present in a concentrationof between about 0.5 and about 10 percent, by weight of said fibers.

12. The method of claim 8 wherein hydrosol of titanium oxide stabilizedwith acetic acid is used to impregnate said fibers and wherein saidimpregnated fibers are dried below about 300 F. and then calcined toprovide a titanium carbide protective layer bonded to the surface of andwholly enclosing said fibers, the titanium of said carbide being presentin a concentration of between about 0.5 and about 10 percent, by weightof said fibers.

References Cited UNITED STATES PATENTS 679,926 8/1901 Voelker 117-2281,000,761 8/1911 Snyder. 2,282,098 10/1940 Taylor. 2,587,036 2/1952Gemmer et al. 117-106 2,587,523 2/1952 Prescott 117-46 2,597,963 5/1952Winter 117-169 2,615,932 10/1952 Marko et a1. 2,703,334 3/1955 Clough eta1 117-228 X 2,898,235 8/ 1959 Bullotf 117-106 3,011,981 12/1961 Soltes23-20920 X 3,053,775 9/1962 Abbott 252-421 3,061,465 10/1962 Norman etal 1l7-107.2 3,073,717 1/1963 Pyle et a1. 117-106 ALFRED L. LEAVITT,Primary Examiner.

RICHARD D. NEVIUS, Examiner.

A. GOLIAN, Assistant Examiner,

1. A METHOD OF IMPROVING THE OXIDATION RESISTANCE OF AMORPHOUS CARBONFIBERS, WHICH METHOD COMPRISES COVERING THE SURFACES OF CARBON FIBERSWITH AND BONDING TO SAID SURFACES A LAYER OF HEAT DECOMPOSABLE ORGANICMATERIAL CONTAINING CARBON AND METAL SELECTED FROM THE GROUP CONSISTINGOF ZIRCONIUM, CHROMIUM, NICKEL, TITANIUM, AND MIXTURES THEREOF, SAIDCARBON FIBERS HAVING BEEN HEAR SHRUNK AT ABOUT 1800-2200* F., ANDTHEREAFTER DECOMPOSING SAID MATERIAL UNDER NON-OXIDIZING CONDITIONS ANDAT ELEVATED TEMPERATURES TO CARBINE OF SAID METAL WHILE SUBSTANTIALLYPERSERVING THE CARBON OF SAID FIBER, SAID TEMPERATURE BEING BELOW THATAT WHICH GRAPHITIZATION OCCURS, WHEREBY SAID FIBERS HAVE IMPROVEDOXIDATION RESISTANCE AT TEMPERATURES IN EXCESS OF 800* F.